As mentioned before, will weld two tanks together. First will fabricate the lower pan
and then just weld the existing GS tank on top. Will try to fit in ventilation and overflow
pipes also as it was in VFR original tank.

There ya go....nice tank fab. PLEASE let us know how many gallons/liters it holds when you are done.

I stated earlier that I bought a Rockster tank to use the base, like you did with the VFR tank, but I also have a 1986 R80G/S BIG Paris/Dakar style tank, like the ones that are sometimes signed by Gaston Rahier. I am thinking of cutting out the tunnel on that and over laying the shell onto the Rockster tank base....a Vintage P2D 8 or 9 gallonish tank on an R1100 custom....someone will probably shoot me for ruining a perfect condition P2D tank, but You can still find them, it is not like they are 1 in a million...yet.

i own a 4th gen vfr750 and the motor is sooooo smooth, sounds brilliant, and really broad spread of power from idle till about 9k rpm.
this bike will be the best dual sport ever.
good luck, cant wait to see it goin.

__________________
If you don't know where you are going, any road will get you there.
-Lewis Carroll.

To test for leaks you can get a big tub like those ones people use for camping gear, and fill it mostly full with water, push the tank down into the water and hold it so the filler is just above the water line. Hold it there for a couple minutes, pull it out and shake it to see if there is any water inside. If you mark the water line you can check how much fuel it will hold by using the displacement method. After you remove the tank pour pre measured amounts of water back in until it rises back up to the line and that is how much it will hold...minus a bit for the metals volume.

That or just go to the tried and true, fill it with water and let it sit...watch for leaks. Measure the water that pours out.

The only issue with that method though is water inside the tank can be a pain to get out sometimes.

Looking fantastic though. Keep us posted with more pics of that nice work. Too bad you are on the other side of the world, I could probably use a bit of help here and there on my beast project.